1. Field of the Invention
The present invention relates to low structure carbon blacks and methods of producing them.
2. Description of the Related Art
Carbon blacks are used as pigments in a wide variety of media, including, for example, toners, inkjet inks, and inkjetable color filters. The structure and surface area of these blacks are chosen to permit a particular loading level of carbon black in a matrix and to reduce conductivity and charge accumulation in the media. Increased loading level increases the optical density (OD), a measure of the opacity of a material, of the media but also increases the viscosity of the coating compositions used to produce the media. Decreasing the structure of the component carbon black can decrease viscosity, allowing thinner layers of the media to be deposited without defects, or it can allow more carbon black to be incorporated at a given viscosity, resulting in a higher optical density.
One method of controlling the structure of a furnace carbon black is by adding alkali ions to a furnace while burning a carbonaceous feedstock. For example, in U.S. Pat. No. 5,456,750, increasing the addition of potassium to a carbon black furnace decreased the structure of the resulting black. However, the resulting metal component in the carbon black can contribute to increased conductivity, and non-carbon materials in the media do not contribute to optical density. In addition, while optical density or tint may be increased by increasing surface area, it becomes increasingly difficult to decrease the structure as the surface area is increased. For example, in the '750 patent, those carbon blacks having the lowest DBP (dibutyl phthalate absorption) also had relatively lower tint than carbon blacks with DBP greater than 40. The lower tint means that greater concentrations of the carbon black in a particular media would have been required to obtain a desired optical density, increasing the cost of the media and increasing manufacturing difficulty, since higher concentrations of carbon black are often more difficult to disperse. Furthermore, as potassium and other metal elements are added to the furnace, the resulting black has more charged groups on the surface and is thus more acidic and hydrophilic. More hydrophilic or acidic blacks (e.g., pH less than 6) may not be compatible with as wide a range of polymers and other components that would be otherwise desirable for use in coating or printing applications. Thus, it is desirable to identify a method of producing carbon blacks having low structure but also having low amounts of alkali metals and low acidity and hydrophilicity and that do not compromise optical density and viscosity in coating, toner, or ink compositions incorporating the carbon black.